Proteus, the second largest moon of Neptune next to Triton, was
discovered by the Voyager 2 spacecraft and Stephen P. Synnott in
1989 thirty three years after the discovery of a significantly
smaller moon, Nereid. It was missed by ground telescope observations
for a couple of reasons. One, it orbits so close to Neptune, that
the reflective glare of the planet itself obscured it. Second,
Proteus is the darkest object in our solar system. Reflecting only
about 6% of the sunlight that strikes it, it is as dark as Saturn’s
dark moon, Phoebe.

Phoebe and Proteus, given their strikingly similar dark
characteristics, are thought by scientists to have formed in the
same part of the solar system and are close to one another in age.
After formation, it is thought that they were each captured by the
gravitational pull of the two planets, Saturn and Neptune.

It has a pro-grade orbit; this means that it orbits Neptune in the
same direction as the planet’s rotation. It takes Proteus 27 Earth
hours to orbit Neptune, indicating its close equatorial trajectory.
Its irregular shape appears to be unaltered geologically. No other
moon in the solar system is as large as Proteus while still having
an irregular, non-spherical shape. Astronomers say that it is as
large as it can possibly be without its own gravitational pull
forcing it into a spherical shape. It is heavily cratered and photos
of the moon give it the appearance of the top of a head of
cauliflower. Its craters are thought to have been formed by impact
of asteroids or comets.

Proteus is over 400 kilometers in diameter, making it about
one-ninth as wide as Earth’s own moon. In keeping with the
mythological theme of nomenclature, Proteus is named for the shape
shifting sea god son of Poseidon – Poseidon being the Greek god of
the sea. Perhaps Proteus is named after this particular sea god
because of its irregular shape. Proteus of mythology was said to
have the ability to change his shape at will. One of the craters of
Proteus is named Pharos – the name of the island where the
mythological sea god was said to reign.

No future plans are made to send spacecraft on a voyage that will
pass Neptune in the future. The data and photos gathered on the
journey in 1989 is likely the last information that will be
available for decades, possibly.